I hope you've already heard back, but I'm guessing it had nothing to do with the content. In our system, "x" means excused, while "z" means zero due to not being received (missing assignment). If a child really gets a zero, it would just be recorded as 0 points, but that's pretty unlikely. The "z" can be handed in one day late for 20% off, or two days late for 50% off, while the "x" does not need to be made up. I think this is a pretty common grading system.